a) Field of the Invention
The invention is directed to an arrangement for generating a chemically active jet (hereinafter: active gas jet) by means of an electrically generated plasma in a process gas being used. The invention is suited particularly for the treatment of surfaces, e.g., for pretreating and cleaning surfaces prior to gluing, coating or painting, for coating, hydrophilization, removal of electric charges or sterilization and for accelerating chemical reactions.
b) Description of the Related Art
Known arrangements for pretreating surfaces of workpieces by means of a gas which is activated in an electric discharge zone are shown in DE 195 46 930 C1, DE 195 32 412 A1 and EP 03 05 241. In patent DE 195 46 930 C1, a whirling flow of the gas to be activated is guided through an electric discharge zone which is formed between a conical center electrode and a ring electrode located externally at the end of a nozzle.
Another, similar method is described in DE 195 32 412 A1 in which the gas to be activated is initially introduced and activated in a whirling flow in the area of a discharge zone occurring along the axis of a cylindrical nozzle pipe with an internally insulated cylindrical outer electrode and a coaxial center electrode and, at the outlet of the discharge zone at which the nozzle pipe narrows in the form of a circular terminating surface of the cylindrical outer electrode, the gas jet is essentially discharged at the terminating surface of the outer electrode.
The solutions mentioned above are disadvantageous in that the gas jet exiting from the nozzle has a considerable electric potential with a value between the potential of the grounded ring electrode and that of the center electrode. With a correspondingly high throughput of gas through the outlet opening of the gas flow, discharge brushes arch out of the nozzle in the direction of the active gas jet in addition. The disadvantage mentioned above limits possible applications of the two solutions mentioned above a) because of the risk of electric shock for the operating personnel and b) because of the possibility of defects induced by electromagnetic fields during surface treatment of sensitive materials, e.g., semiconductor substrates which may also have doped layers or structures.
According to EP 03 05 241, the gas to be activated is guided directly through an electric discharge zone. The discharge zone is formed in a pipe by means of an electric field, wherein either electrodes are arranged laterally within the pipe successively in the flow direction of the gas or a discharge chamber which is installed in a waveguide and which comprises insulating material without electrodes is provided. This solution has the above-mentioned disadvantage that at a high velocity of the activated gas flow there is a high probability that the electromagnetic fields and the electric discharge zone itself will exit from the discharge chamber in the direction of the active gas jet due to the total absence of a shielding ring electrode at the end of the discharge chamber. The arrangement described in EP 0 305 241 A1 protects operating personnel by means of a separate, closed treatment chamber in which the surface treatment of the material takes place. The resulting complicated conditions for material processing are disadvantageous and, if the protective chamber were omitted, would lead to an uncontrolled change in the process conditions and endangerment of operating personnel.
All of the technical solutions mentioned above are characterized in that the velocity, temperature and geometry of the active gas jet are determined by the electrical, thermal and gas-dynamic conditions necessary for the formation and ignition of the electric discharge zone for gas activation. However, these conditions for gas activation in an electric discharge zone do not always prove to be the optimal conditions for surface treatment by means of the active gas jet.
For example, use of an electric discharge at atmospheric pressure and of the resulting temperatures higher than 5000 K for surface treatment is very problematic because the majority of materials to be processed do not withstand such temperatures. Another problem is posed for the electric discharge zone by high process gas velocities, e.g., supersonic velocity, because these velocities can be maintained under highly dynamic conditions only with the greatest difficulty. However, the above-mentioned uses of the active gas jet require higher gas throughput in order to reduce the time within which the active gas jet reaches the surface to be treated proceeding from the discharge zone, since the loss of activity of the gas jet is effectively reduced by reducing the recombination processes.